The primary objectives of this study are to evaluate the safety of multiple IM doses of SB209763 in infants and young children with underlying diseases which predispose them to serious RSV infection, to evaluate the efficacy of multiple IM doses of SB209763 in infants and young children as prophylaxis against the development of serious RSV infection, that is, serious enough to require hospitalization and to evaluate whether antibodies to SB209763 are formed in infants and young children given multiple IM doses.